On the Search for Quasar Light Echoes

The UV radiation from a quasar leaves a characteristic pattern in the distribution of ionized hydrogenthroughout the surrounding space. This pattern or light echo propagates through the intergalacticmedium at the speed of light, and can be observed by its imprint on the Lyα forest spectra of background sources. As the echo persists after the quasar has switched off, it offers the possibility of searching fordead quasars, and constraining their luminosities and lifetimes. We outline a technique to search forand characterize these light echoes. To test the method, we create artificial Lyα forest spectra from cosmological simulations at z = 3, apply light echoes and search for them. We show how the simulationscan also be used to quantify the significance level of any detection. We find that light echoes from thebrightest quasars could be found in observational data. With absorption line spectra of 100 redshiftz ∼ 3− 3.5 quasars or galaxies in a 1 square degree area, we expect that ∼ 10 echoes from quasars withB band luminosities LB = 3 × 10 45 ergs exist that could be found at 95 % confidence, assuming aquasar lifetime of ∼ 10 yr. Even a null result from such a search would have interesting implicationsfor our understanding of quasar luminosities and lifetimes. Subject headings: Cosmology: observations – large-scale structure of Universe